Window Rough Opening Size Calculator

A rough opening (RO) is the framed opening in a wall into which a window unit is installed. The rough opening must be larger than the window unit itself to allow for shimming, leveling, and insulation.

Why Rough Openings Are Larger Than Windows

When a window is manufactured, its dimensions refer to the overall frame size. The rough opening adds:

• Width: Add 50 mm (2") to the window width — 25 mm (1") on each side for shimming
• Height: Add 50–75 mm (2–3") to the window height — allows for shimming at the sill and a nailing flange at the top

The exact addition varies by manufacturer. Always check the window’s installation guide for the specified RO. If not available, the standard rule is:

Rough Opening Width = Window Width + 50 mm (2") Rough Opening Height = Window Height + 63 mm (2.5")

Egress Windows

For windows in bedrooms and basement sleeping areas, building codes in North America require minimum egress dimensions so occupants can escape in a fire. The International Residential Code (IRC) requires:

• Minimum net clear opening area: 5.7 ft² (0.53 m²)
• Minimum clear opening height: 24 inches (610 mm)
• Minimum clear opening width: 20 inches (508 mm)
• Maximum sill height from floor: 44 inches (1,118 mm)

Note: The “net clear opening” is the openable area of the window, not the frame size. Most window manufacturers publish the net clear opening for their windows.

Framing Components

A window rough opening is framed with:

• King studs: Full-height studs on each side
• Jack studs (trimmers): Shorter studs supporting the header
• Header: Structural beam across the top of the opening
• Rough sill: Horizontal framing at the bottom of the opening
• Cripple studs: Short studs above and below the opening

This calculator gives the raw rough opening dimensions only.